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Sleeping position and risk of stillbirth

Covered widely in the news this week were the results of a major study into stillbirth, the death of a baby in the last trimester before it is born. In the UK, there are over 3,000 stillbirths every year, which equates to around nine every day – or around 1 in 225 pregnancies. Though the absolute risk of stillbirth is low, it’s a tragedy for anyone who experiences it, and a terrifying prospect for many expectant parents.

As a result, researchers are working hard to try to understand what causes stillbirth, why stillbirth rates vary across the world, and ultimately what can be done to try to prevent as many stillbirths as possible.

The Midlands and North of England Stillbirth Study (MiNESS)

The results were based upon a survey of over 1,000 women in the Midlands and North of England – 291 had recently experienced a late stillbirth (after 28 weeks), the other 733 had not. Amongst a variety of different factors, one of the questions the women were asked was about which position they had gone to sleep in – for example, on their side, tummy, or back.

The researchers found that pregnant mums who had gone to sleep on their back were more than twice as likely (2.3-fold) to experience late stillbirth as those who had gone to sleep on their side.

This is not the first time such a link has been suggested. A New Zealand study in 2011 first identified a potential link between sleep position and stillbirth, and since then, three further studies – in 2013, 2015 and earlier in 2017 – also found this link. These studies were relatively small and so couldn’t necessarily be considered solid proof of this effect- but MiNESS is the largest and probably the most reliable study to date.

So why should sleeping position affect risk of stillbirth? It is not entirely clear, as there could be many factors at play. One theory is that if a pregnant mother sleeps on her back, the baby can put pressure on a major blood vessel in the torso, the inferior vena cava, which returns blood back from the lower part of the body to the heart. The idea is that the restriction of the inferior vena cava could reduce the amount of blood that the heart can then pump back to the rest of the body – and therefore the unborn child, who depends entirely on their mother’s blood supply for oxygen.

Diagram showing the position of the inferior vena cava, a blood vessel which in theory could be affected by the weight of an unborn baby when the mother is sleeping on her back.

What about other risk factors for stillbirth?

How does sleeping on your back compare with other known risk factors? As well as identifying that back sleeping increased risk of stillbirth by 2.3-fold, the researchers also confirmed the effects of some known lifestyle risk factors. They calculated that in this study, smoking and obesity increased risk of stillbirth by 2-fold and 1.7-fold, respectively (roughly in line with previouslarge studies of risk factors for stillbirth). Therefore, as far as the effect on stillbirth risk is concerned, sleeping on your back is comparable to other well-known risk factors.

Another way to compare risk factors is to look at how many stillbirths can be attributed to each ‘cause’. Note that any one stillbirth can be caused by a range of different factors, so it’s difficult to say with complete certainty what ‘caused’ any individual stillbirth. However, by knowing how many people are exposed to different risk factors, and how strongly those factors affect risk, it is possible to estimate what proportion of stillbirths can be attributed to each factor.

The MiNESS researchers estimate that sleeping on the back accounts for 3.7% of stillbirths, which equates to roughly 130 stillborn babies each year in the UK. Because a significant proportion of pregnant mothers in this study smoked (17.3%), they estimated that smoking accounted for more late stillbirths: 14%, or nearly 500 stillbirths each year. So in this sense fewer stillbirths are ‘caused’ by sleeping on the back than by smoking.

Preventing stillbirths

So with our current knowledge of what increases risk of stillbirth, how might we go about trying to prevent as many as possible from happening? Of course, making lifestyle changes like stopping smoking or losing weight is difficult for anyone, including pregnant mums. The NHS runs programmes to encourage mums-to-be to stop smoking, and though it’s encouraging that such programmes can work – like an evaluation of one in the North East of England in earlier this year demonstrated – interventions to encourage people to make major changes to their lifestyle are unlikely to be 100% effective.

But could an intervention to encourage women to sleep on their sides be successful? On the same day as the results from MiNESS were released, the charity Tommy’s launched their “Sleep on Side” campaign, endorsed by NHS England, the Royal College of Obstetricians and Gynaecologists, the Royal College of Midwives and the Royal College of GPs. Sands, the Stillbirth & Neonatal Death charity, have also updated their recommendations to pregnant women.

It’s a simple and clear message, but will this campaign be effective at reducing rates of stillbirth? On the one hand, unlike losing weight and stopping smoking, sleeping on the side is a relatively simple change that most pregnant women will probably be willing and able to make. Anecdotally, researchers involved in the 2017 New Zealand study I mentioned earlier found that, after the link between sleep position and stillbirth was first identified in 2011, already fewer pregnant women are sleeping on their back during their final trimester.

Nevertheless, it’s difficult to predict how large an impact campaigns like this will make. Amongst the women studied in MiNESS, only a small percentage (3.3%) slept on their back anyway, so there are far fewer women to reach than stop-smoking services. And although a formal large-scale trial of a side-sleeping intervention could definitively answer the question, the MiNESS researchers estimate such a trial would need to recruit at least two million women – which is an unfeasibly large number.

Nevertheless, if sleeping on the side could prevent 130 stillbirths each year in the UK, many people would consider it a message worth spreading. As pointed out by Michelle Cottle who writes the blog ‘Dear Orla’ about her experience of stillbirth, a message like this can help pregnant women regain some sense of control at what can be an uncertain time – and if it saves even one life, and spare the trauma inflicted by stillbirth, then it’ll be worth it.